The present invention relates to acoustic ink printers, and, more particularly, to ink transport systems for such printers.
In acoustic ink printing an array of ejectors, forming a printhead, is covered by pools of liquid ink. Each ejector can direct a beam of sound energy against a free surface of the liquid ink. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. If the radiation pressure is sufficiently high, individual droplets of ink are ejected from the liquid surface to impact upon a sheet of medium, such as paper, to complete the printing process.
Typically, the ejectors are arranged in a linear array so that the ejectors are aligned perpendicularly to the movement of the recording medium which receives the ejected ink droplets. Alternatively, the ejectors may be arranged in an array of rows and columns, with the rows stretching across the width of the recording medium and the columns of ejectors approximately perpendicular along the movement of the recording medium. Often the columns of ejectors are not arranged exactly perpendicular to the ejector rows, but at oblique angles with the rows. In other words, the ejector rows of the array are staggered.
Each ejector for an acoustic ink printer must be supplied with ink and a good ink supply system should maintain a flow of ink constantly. A flowing ink supply system cools the ink and stabilizes the ink temperature more easily. Additionally, the flowing ink supply system keeps the ink free of various contaminants, such as paper dust which might settle upon the free surfaces of the ink, by sweeping the contaminants away. The constantly flowing ink also maintains fresh ink to the free surfaces. Without the constant flow of ink, the differing evaporation rates of the constituents within the ink may adversely affect the uniformity of the ink composition associated with each ejector and therefore, the uniformity of performance of the ejectors.
Ideally, each ejector when activated ejects an ink droplet identical in size to the droplets of all the other ejectors in the array. Thus, each ejector should operate under identical conditions.
One problem which arises particularly with an ink supply of flowing ink is the equalization of the hydrostatic pressure of the free surfaces associated with each ejector. Equalization may be relatively simple with a small number of ejectors, but as the number of ejectors increases in higher-performance and higher-resolution printers, the ink supply system for delivering ink to the ejectors becomes more complex and the equalization of pressure at each ejector more difficult. For example, acoustic ink printers having resolutions finer than 300 dots per inch, the present standard for laser printers, are now under consideration with the attendant problems of complexity in the ink transport system. Nonetheless, despite the increased complexity, the ink supply system must maintain equal hydrostatic pressures at the free surfaces of each ejector.
The present invention solves or substantially mitigates this problem of hydrostatic pressure equalization of the free surfaces of each ejector in an acoustic ink printer with an ink transport system which maintains the ink under constant flow.